FIG. 1 is an exploded cross-sectional view of a prior art circuit board test fixture 100. Test fixture 100 includes test head 102, probe plate assembly 104 and protective plate 106.
Test head 102 is equipped with numerous test head pins 110 that are mounted to test head platform 112. For ease of illustration, the number of test head pins shown in the drawing is intentionally low. An actual test fixture would more commonly contain one or two thousand test head pins, and a high-node-count test fixture may contain perhaps three thousand or more test head pins. Each of test head pins 110 is a spring-loaded probe-and-socket assembly well known in the industry. Typically, the spring probes selected for each of test head pins 110 might be capable of exerting a force of perhaps four ounces. Test head pins 110 are electrically coupled to test head control system 114. Test head control system 114 provides generic test resources such as power, ground and programmable signal sources sufficient to verify the functionality of a wide variety of circuit board types.
Probe plate assembly 104 includes probe plate 116, frame 118 and alignment plate 120. Frame 118 is typically made of aluminum, steel or other metal. Probe plate 116 is typically made of type G10 fiberglass. Alignment plate 120 is typically made of plastic. Numerous probe pins 122 are mounted to probe plate 116. Probe pins 122 are also conventional spring-loaded probe-and-socket assemblies. However, the spring force selected for probe pins 122 usually would be somewhat higher than the spring force selected for test head pins 110. For example, the spring probes selected for each of probe pins 122 might be capable of exerting perhaps eight ounces of force. Numerous personality pins 124 are also mounted to probe plate 116. Personality pins 124 are not spring loaded, but have a mounting member and a conducting member. The mounting member is typically installed in probe plate 116 using an interference fit. The conducting member extends from the mounting member and is operable to make contact with a test head pin 110. Although the conducting member is substantially rigid along its own axis, it is capable of moving radially to some degree away from the axis of the mounting member. This movement enables personality pins 124 to pass through holes in alignment plate 120 that are not perfectly collinear with the axis of the personality pin mounting member. Further background relating to personality pins 124 may be found in U.S. Pat. No. 4,799,007, titled "Bendable Pin Board Test Fixture," invented by Stephen J. Cook and Michael L. Bullock, assigned to Hewlett-Packard Company and hereby incorporated by reference in its entirety. The mounting location of personality pins 124 on probe plate 116 is dictated by the arrangement of the various test resources that are permanently connected to test head pins 110. The mounting location of probe pins 122, on the other hand, is dictated by the arrangement of test nodes 128 on device-under-test ("DUT") 108. Probe pins 122 are electrically coupled to personality pins 124 typically by wire wrap or solder connections 126. Numerous alignment rods 130 (typically tooling pins) are also mounted to probe plate 116. Alignment rods 130 provide guidance for the proper installation of protective plate 106 and DUT 108 onto test fixture 100.
Vacuum source 132 is coupled to test fixture 100 via valve 134. Channels 136, 138 and 140 are provided within test head platform 112, frame 118 and probe plate 116, respectively, in order to communicate the vacuum to the underside of protective plate 106. Oversized probe pin holes 142 in protective plate 106 communicate the vacuum to the underside of DUT 108, causing it to be pulled firmly down onto test fixture 100 when the vacuum is applied. The result is that probe pins 122 engage test nodes 128. Gasket material 144 is provided on both sides of protective plate 106 to complete the vacuum seal. Vacuum is only one method commonly used to engage DUT 108 with probe pins 122. Other known methods include the application of force from the top side of DUT 108 using pneumatic, hydraulic or motor-driven devices. Once DUT 108 has engaged probe pins 122, test head control system 114 can automatically exercise DUT 108 as necessary to verify its functionality.
Further background relating to prior art circuit board test fixtures may be found in U.S. Pat. No. 4,771,234, titled "Vacuum Actuated Test Fixture," invented by Stephen J. Cook and Kris J. Kanack, assigned to Hewlett-Packard Company and hereby incorporated by reference in its entirety.
Test fixtures like test fixture 100 work well when the number of test head pins 110 is low to moderate--for example, less than 3,000 nodes. In systems having a high node count, however, problems arise because of the aggregate force exerted by test head pins 110 on probe plate 116 via personality pins 124. Consider the case of a test head having a node count of 5,000 nodes: If each test head pin exerts a force of four ounces, the aggregate force applied to probe plate 116 by personality pins 124 will equal 1,250 pounds. FIG. 2 illustrates the problems caused by such large forces. While probe plate 116 may be able to withstand the force without breaking, probe plate 116 nevertheless begins to bow up towards protective plate 106 and DUT 108. This bowing causes some of the probe pins, such as probe pin 122A, to be closer to DUT 108 than other probe pins, such as probe pins 122B and 122C. In addition, the bowing of probe plate 116 causes the probe pins and alignment rods to fan out radially. These effects cause numerous problems. First, it can become difficult to mount DUT 108 onto test fixture 100 because the ends of alignment rods 130 are displaced from their original positions. If DUT 108 can be mounted onto the alignment rods, then it may yet be difficult to unmount DUT 108 because the alignment rods may bind against the sides of the alignment rod holes within DUT 108. Second, the bowing of probe plate 116 and the consequent raising of the center-most probe pins makes it difficult to maintain the vacuum seal of the test fixture. Third, in extreme cases of bowing such as that shown in FIG. 2, the probe pins may miss their intended targets as indicated at 146, 148 and 150. These problems get worse as the node count increases and as the size of the test fixture increases.
Although attempts have been made to strengthen probe plate 116 by reinforcing it or making it thicker, such attempts have generally led to more expensive and cumbersome embodiments and have not successfully prevented the just-described bowing problem. Moreover, it is sometimes desirable to use a transparent plexiglass material for probe plate 116 so that operators may see into the test fixture. Plexiglass is significantly less strong than opaque fiberglass materials such as type G10 fiberglass. Therefore, with high-node-count test fixtures such as test fixture 100, plexiglass may not be used to form probe plate 116 because it exacerbates the bowing of probe plate 116.